Rapid and continuous chemical transmission between neurons requires that synaptic vesicles be recycled after fusing with the plasma membrane to release transmitter. Endocytosis, the process by which neurons reinternalize fused vesicular membrane, is the first essential step in this recycling process. Despite its clear necessity in maintaining an available pool of synaptic vesicles and balancing the size of a presynaptic terminal against membrane added during exocytosis, very little is known about endocytosis. This is especially true in neuronal systems. The proposed study seeks to characterize endocytosis from a physiological perspective using membrane capacitance measurements of single presynaptic bipolar cell terminals from the goldfish retina. The specific aims of this project are: 1) To characterize the effect of neurotransmitters on the kinetics of endocytosis, 2) To determine the effect of phosphatase inhibition on endocytosis in the presynaptic terminal, 3) To determine the quantitative relationship between membrane addition and retrieval during ongoing exocytosis and endocytosis. To measure endocytosis in the intact bipolar cells of a retinal slice. The results are expected to provide fundamental insight about presynaptic signal modulation in neurons and further elucidate the functional characteristics of retinal bipolar cells and their role in visual signal transduction.